We are requesting funds to purchase a new Gammacell 220 Excel Self-contained High Dose Rate Research Irradiator as a shared instrument for biomedical research at MIT. The instrument represents the centerpiece of a radiation facility for state-of-the-art studies of the effects of radiation on biological systems. In addition to the Principal Investigator, there are eight Co-Applicants who have a critical need for a high dose-rate radiation source for studies ranging from stem cell research, immune function, and neurobiology, to toxicogenomics, genetic toxicology, and biochemistry. The critical need for this device arises directly from the fact that the high dose-rate Gammacell will allow us to pursue studies that would otherwise not be possible. The acquisition of the Gammacell will provide a valuable resource not only for the scientists participating in this grant application, but also for faculty and students across the MIT campus and in the Boston/Cambridge scientific community, since there exists no similar device in the area. In addition to the disciplines represented by the Principal Investigator and the Co-Applicants, the Gammacell radiation source will also be of interest to MIT scientists with non-NIH funding in the Departments of Materials Science and Engineering (radiation effects on novel materials used in packaging, medical disposables, military uniforms, space vehicles, etc.), Aerospace Engineering (qualification of space-bound electronics), and Nuclear Engineering (dosimetry calibration).